Add support for an encoding fallback.

Added a FallbackEncoding property to IrcConnection. If IrcConnection's bytes-to-string decoding fails using the encoding stored in Encoding, decoding using FallbackEncoding is attempted. (The FallbackEncoding will, very probably, fail silently.)

The default FallbackEncoding is Encoding.Default, i.e. the same as Encoding. That makes this feature opt-in.